1. Field of the Invention
The present invention relates to a loop-type directional coupler having a waveguide, and in particular a hollow waveguide, a planar waveguide or a co-axial waveguide, in the form of a half-loop antenna which has a first arm and a second arm, for the contactless coupling-out of a forward signal “a” on a waveguide and a backward signal “b” on said waveguide.
2. Description of Related Art
It is known for what are termed directional couplers to be used to determine and separate forward radio frequency current and voltage waves “a” and backward radio frequency current and voltage waves “b” on a guide or line and to determine the voltage “U” and current “I” on the guide or line. The directional coupler is one of the most widely used components in radio frequency and microwave circuits. It is a reciprocal four-port component in which, in the ideal case, two ports are decoupled from one another when all the ports have reflection-free terminations. For example, let it be assumed that port 1 is the input port to which a signal is fed. Let all the ports have reflection-free terminations. Port 4 for example is then the isolated port to which no part of the infed power is coupled. The other two ports are called the transmitted port and the coupled port.
An important variable for defining the quality of a directional coupler is its sharpness of directivity (directional coupling) or simply directivity. Sharpness of directivity is the ratio of the power at the coupled port to the power at the isolated port when all the ports have reflection-free terminations. According to K. W. Wagner, “Induktionswirkung von Wanderwellen in Nachbarleitungen” [Inductive effect of travelling waves on neighboring lines], Elektrotechnische Zeitschrift, Vol. 35, pages 639-643, 677-680, 705-708, 1914, the optimum directivity is obtained from a directional coupler comprising two coupled lines when the ratio of the inductive coupling factor to the capacitive coupling factor is equal to the product of the characteristic impedances of the individual lines.
Directional couplers are often used in measuring systems to allow the forward and backward waves to be determined separately. In circuitry technology, directional couplers are used as decoupled power dividers in attenuators, phase-shifters, mixers and amplifiers. The directional couplers are constructed in this case from for example co-axial waveguides, hollow waveguides and/or planar waveguides.
A possible coupling structure for separating the forward and backward waves is the loop-type directional coupler which is described by P. P. Lombardini, R. F. Schwartz, P. J. Kelly in “Criteria for the design of loop-type directional couplers for the L band”, IEEE Transactions on Microwave Theory and Techniques, Vol. 4, No. 4, pages 234-239, October 1956, and by B. Mäher in “An L-band loop-type coupler”, IEEE Transactions on Microwave Theory and Techniques, Vol. 9, No. 4, pages 362-363, July 1961. A loop-type directional coupler comprises a loop of guide which is positioned above or in a waveguide. Any desired waveguides, such as hollow guides, planar strip guides or co-axial guides may be used in this case. There are a wide variety of uses which can be made of a loop-type directional coupler. For example, F. De Groote, J. Verspecht, C. Tsironis, D. Barataud and J.-P. Teyssier, in “An improved coupling method for time domain load-pull measurements”, European Microwave Conference, Vol. 1, page 4 et seq., October 2005, and K. Yhland, J. Stenarson in “Noncontacting measurement of power in microstrip circuits” in 65th ARFTG, pages 201-205, June 2006, use a loop-type directional coupler as a component in a contactless measuring system.
Inductive and/or capacitive coupling structures are employed to determine the scattering parameters of a device under test (DUT) by using a contactless, generally vectorial, measuring system. The current and/or voltage on a signal line or guide which is directly connected to the device under test are determined by means of these coupling structures. Alternatively, the forward and backward waves on the signal line are measured, directional couplers then being used as coupling structures for separating the two waves.
The accuracy of an uncalibrated and a calibrated measuring system for determining the forward and backward waves by means of directional couplers depends on, amongst other things, the directivity of the couplers. When loop-type directional couplers are used in the way which is taken as an example, their directivity can be optimized by means of the positioning and angle of the loop relative to the signal line or guide and by varying the geometry of the loop. However, wide-band optimization of the directional coupling (over a plurality of octaves) is not possible by this means. The geometry of the configuration has to be re-optimized for each frequency range. A very accurate loop positioning unit is required for this purpose, and this causes a tremendous increase in the complexity of the directional coupler.